1. Field of the Invention
The present invention relates to a current driving apparatus, and particularly to a current driving apparatus using PWM (pulse width modulation.
2. Description of Related Art
Referring to FIG. 9, a driver circuit has a load comprising LEDs (light emitting diodes). A current clamping resistor 20 is provided between a power supply 10 and a light emitting diode (LED) array 30 formed in an m×n matrix of light emitting diodes 31. Voltage provided by the power supply 10 is designated U, resistance of the said current clamping resistor 20 is R, resistance of each light emitting diode is Rs, and current in the main path is I. The ohm's law for the driver circuit shown in FIG. 9 can be expressed by the equation:U−m×Von=I(R+Rs(m/n))wherein Von is a threshold voltage of the light emitting diode 31.
Simplifying the above equation, we have:U−Vx=I(R+Rx), wherein Vx=m×Von, Rx=Rs(m/n)
Because of the existence of Von, current I is not in direct proportion to U. For example, when U changes to 2U, current I does not double, but instead changes to less than 2I. Since I do not vary linearly with U, it is difficult to control the current I precisely.
Additionally, if timing control is required, the additional timing control circuit makes the whole circuit more complicated.
It is desirable to provide an improved current driving apparatus which overcomes the above problems.